This is a broadly based continuation of work in progress in this deapartment dealing with the epidemiology and evolution of plasmids in gram-positive bacteria, and with the molecular basis of replication control and incompatibility of the penicillinase plasmids of Staphylococcus aureus. New aspects of the investigation include a molecular cloning analysis of plasmid partitioning and the relation of plamids to the host cell envelope. These studies will explore the mechanism of plasmid curing during protoplast regeneration, which is thought to involve a deficiency in the partition-cell division process due to the absence of the cell wall. They will also analyze the relation of plasmid partitioning to spore transmission as well as the nature of host factors involved in stable plasmid maintenance, particularly as revealed by trans-species plasmid instability. This latter phenomenon will be considered to represent a mismatch between the plasmid and host factors involved in the initation of replication and others involved in partitioning.